1. Field of the Invention
The present invention relates to a circuit for rotating an ultrasonic motor, and more particularly to a circuit for rotating an ultrasonic motor which is capable of optimally rotating an ultrasonic motor.
2. Related Art Statement
Hitherto, a variety of circuits each for rotating an ultrasonic motor have been suggested. For example, U.S. Pat. No. 4,510,411 and U.S. Pat. No. 4,743,788 have disclosed a technology utilizing a fact that the voltage level of an output signal (a monitor signal) from a monitor electrode disposed in an ultrasonic motor becomes maximum at the resonant frequency of the ultrasonic motor so as to monitor the voltage level of the monitor signal, wherein the ultrasonic motor is rotated at the resonant frequency or in a frequency range somewhat higher than the resonant frequency in accordance with the voltage level.
Further, U.S. Pat. No. 4,833,358 has disclosed a technology utilizing a fact that the phase of the foregoing monitor signal with respect to the operation voltage is considerably changed in the vicinity of the resonant frequency to monitor the phase of the monitor signal, wherein the ultrasonic motor is rotated at the resonant frequency or in a frequency range somewhat higher than the resonant frequency in accordance with the monitored phase.
On the other hand, U.S. Pat. No. 4,853,579 has disclosed a technology which is capable of determining the optimum operation frequency for the ultrasonic motor by monitoring the difference between the phase of the voltage of the operation signal supplied to the operation electrode of the ultrasonic motor and the phase of the electric current.
However, the technologies disclosed in U.S. Pat. No. 4,510,411, U.S. Pat. No. 4,743,788 and U.S. Pat. No. 4,833,358 encounter a necessity that the monitor electrode is disposed on the ultrasonic motor, resulting in a problem of excessive labor and undesirable degree of cost reduction.
The technology disclosed in U.S. Pat. No. 4,853,579 enables the monitor electrode to be omitted from the structure so that the problem experienced with the U.S. Pat. No. 4,510,411, U.S. Pat. No. 4,743,788 and U.S. Pat. No. 4,833,358 is overcome. However, if a load is applied on to the ultrasonic motor as shown in FIG. 6 (in the loaded state shown in FIG. 6), the state of change in the phase difference is considerably changed due to the load as compared with the non-loaded state. Therefore, if the foregoing technology is applied to a camera having a lens, such as a photographing lens, the overall body of which is moved forward and the load of which is therefore changed considerably, the load is considerably changed due to the change in the attitude at the time of use, and therefore a problem arises in that the optimum operation frequency cannot easily be followed.